Apparatus and method for starting up a steam generator

ABSTRACT

At start up, about 15 percent of a full working load of feed water is supplied to the evaporator directly while an additional flow of feed water at least 30 percent larger than the flow to the evaporator is supplied to the superheater by the additional feed line under full working pressure. During this time, the flow of working medium is bypassed back into the degasser while the burner is supplied with about 15 percent of full fuel load and 25 percent of full combustion air load. Once steam becomes generated in the evaporator, the pressure at the superheater is rapidly reduced to permit the starting up operation to proceed at an increasing pace.

g Unite States Patent [151 name Doiezall Mar. M, 11972 [54] APPARATUS AND METHOD FOR 3,411,485 11/1968 Kraus ..122/40e STATING UP A STEAM GENERATOR Primary Examiner-Kenneth W. Sprague [72] lnventor. Richard Dolezal, Wlnterthur, Switzerland Attorney xenyon and Kenyon Remy Carr & Chapin [73] Assignee: Sulzer Brothers, LtdL, Winterthur, Switzerland [57] Still [22] Filed: Sept. 11, 1970 At start up, about 15 percent of a full working load of feed water is supplied to the evaporator directly while an additional [21] Appl' 71437 flow of feed water at least 30 percent larger than the flow to the evaporator is supplied to the superheater by the additional [30] Foreign Application Priority Data feed line under full working pressure. During this time. the flow of working medium is bypassed back into the degnsser .2 1 1 ..14 Sept 3 969 Swltzer and 327/69 while the burner is supphed with about 15 percent of full fuel 52 us. Cl ..l22/406 ST and 25 P Of 10m Once Steam [51] lot. (31 ..F22b 29/12 becomes generated in the evaporator, the Pressure at the [58] Field of Search ..122/406 R, 406 s, 406 ST perheater is p y reduced to Permit the starting up p tion to proceed at an increasing pace. [5 6] References CM 10 u, 3 Drawing ri um UNITED STATES PATENTS 2/1968 Knizia ..l22/406 Patented March 14, 1972 3,648,667

2 Sheets-Sheet 1 lnvenlor: QIcr-IAFQD DOl-BZAL.

Patented March 14, 1972 3,648,667

2 Sheets-Sheet f? In venlor:

P/CHAQD Douszaz.

APPARATUS AND METHOD FOR STARTING UP A STEAM GENERATOR This invention relates to an apparatus and method for starting up a steam generator.

Heretofore, steam generators which have been constructed with a superheater comprising a plurality of tubes connected in parallel with respect to the flow of a working medium thereby, a bypass line connected to the outlet from the superheater, and a pressure maintaining element within the bypass line have required a lengthy process for starting up. This has been due to the limits imposed on the increase of the firing rate since an excessively rapid increase of the firing rate may cause the water in some of the parallel connected tubes of the superheater to be driven out earlier than in other tubes. If such should occur, there is some danger that the working medium remaining in these latter tubes can stagnate and cause fractured tubes.

Accordingly. it is an object of the invention to permit a rapid start up of a steam generator.

It is another object of the invention to provide for a start up of a steam generator in which stagnation of the working medium in the superheater tubes of the steam generator is minimized.

Briefly, the invention provides a method and apparatus for starting up a steam generator in a manner to permit a rapid increase in the firing rate.

The method of the invention involves the steps of supplying a flow of feed water to the'inlet of a steam generator for flow first through the tube system of the steam generator and then through a bypass line, and supplying, from a time no later than when the fire in the combustion chamber of the steam generator is ignited, additional feed water to the superheater of the steam generator tube system while bypassing the evaporator of the steam generator tube system. This additional feed water is fed at a rate of flow at least 30 percent larger than the rate of flow of the feed water supplied to the inlet of the system generator. In addition, the method includes the subsequent step of suddenly and substantially reducing the additional supply of feed water to the superheater while greatly diminishing the pressure at the outlet end of the superheater by opening a pressure maintaining element in the bypass line. In this way, a rapid increase of the firing rate is made possible without greatly increasing the risk to the heating surfaces so that the starting up procedure can be shortened.

The additional supply of feed water to the superheater results in a continuous flow of feed water in the superheater so that, during this period of time, the generation of steam in the superheater is prevented. Accordingly, the steam generator can be started at the beginning of the starting up procedure with a higher firing rate or the firing rate can be increased earlier or to a greater extent than previously without excessive thermal stresses being applied, particularly to the tubes of the superheater. At an appropriate stage when the enthalpy of the feed water at the outlet from the evaporator will have reached a sufficiently high value, there is a sudden and substantial reduction in the additional supply of feed water to the superheater and a simultaneous opening of the pressure maintaining element. This results in a sudden generation of steam in the evaporator owing to the pressure reduction, so that the water in the superheater is blasted out. This is followed immediately by cooling of the superheater by the inflowing steam. The further stages of the starting up procedure will then take place in the usual manner.

The apparatus of the invention is adapted to cooperate with a steam generator having a feed water inlet line, an evaporator, a plurality of tubes connected in parallel with respect to the flow of a working medium such as feed water, in order to function as a superheater, a bypass line connected to the outlet from the superheater and an adjustable pressure maintaining element in the bypass line. The apparatus includes an additional feed water line which enters the working medium flow system between the evaporator outlet and the superheater inlet and is constructed to carry a flow which is equal to at least percent of the rate of flow of feed water at full load.

Since the additional feed water line is constructed to carry a flow which is equal to at least 20 percent of the rate of flow of feed water at full load there is no risk of confusing the line with a conventional injection line, serving for temperature regulation, since such an injection line would be dimensioned for only approximately 10 percent of the full-load feed water flow rate. By contrast to a known injection apparatus in which a valve in the pipeline is controlled by a signal related to superheater stern temperature, flow through the additional feed water line will normally be controlled by a valve operated by hand or by an automatic control system controlling the starting up operation.

These and other objects and advantages of the invention will become more apparent from the following detailed description and appended claims taken in conjunction with the accompanying drawings in which:

FIG. I schematically illustrates a steam generator provided with an apparatus according to the invention;

FIG. 2 illustrates in part a modified steam generator according to the invention; and

FIG. 3 illustrates in part a further steam generator modified according to the invention.

Referring to FIG. 1, the steam generator 1 includes a pres sure vessel 2, the lower part of which is constructed as an evaporator 4 having interwelded, finned tubes and enclosing a combustion chamber 3. In addition, a superheater 5 comprising a plurality of tubes which are connected in parallel with respect to a flow of steam through the generator 1 is positioned in the pressure vessel above the combustion chamber 3. For the sake of simplicity only one tube is shown. The lower end of the pressure vessel 2 is provided with a burner 6 which leads into the combustion chamber 3 and is supplied through a duct 8 with compressed combustion air and through a duct 7 with fuel, for example, oil. A manifold 21 is constructed to the outlet end of the superheater surface 5 from which superheated steam can enter a steam turbine 23 via a valve 22. The outlet of the steam turbine 23 is connected by a duct 24 to a mixing position 25 at which partially expanded steam is mixed with the flue gases which are thus cooled. The upper end of the pressure vessel 2 is connected to a duct 26 which leads to a gas turbine 27 in which the mixture of partially expanded steam and flue gas is expanded. The outlet of the gas turbine 27 is connected by a duct 28 to a chimney. The steam turbine 23 and the gas turbine 27 together with a compressor 10 for the combustion air and an electricity generator 9, which can be adapted to operate as a motor, are mounted on a common shaft.

A bypass line 311, leading to a degasser 16 and containing a pressure maintaining element 32 branches off between the manifold 21 and the valve 22. A feed waterline 11 which leads from the degasser 16 to the inlet of the evaporator 4 in order to supply a flow of feed water containsa feed water pump 15 and a flow control valve 17. A line 34 branches from the feed water line 11 and connects to a mixing vessel 20 into which the working medium leaving the evaporator 4 is discharged and with which the pipes of the superheater 5 communicate via a manifold 21'. The duct 34 is provided with a valve 35 and is so dimensioned that at least 20 percent of the full-load feed water flow rate is able to flow therethrough. The degasser 16 is supplied with water through a demineralizing apparatus 30.

When the steam generator plant is started up, the feed water pump 15 is set into operation and I5 percent of the full-load feed water flow rate is supplied to the steam generator 1 via the control valve 17. The valve 22. upstream of the turbine 23 is closed and the pressure retaining element 32 is slightly opened so that, after flowing through the evaporator 4 and superheater 5, the feed water can be transferred into the bypass line 31. The superheater surface 5 is additionally supplied with a quantity of feed water through the duct 34 and the opened valve 35. This quantity amounts to approximately 35 percent of the full-load feed water flow rate. Allso, the pressure retaining element 32 is adjusted as to provide a full-load working medium pressure of the order of atm. gauge. The burner 6 is then fed with fuel at a rate corresponding approximately to 15 percent of full-load and with air corresponding approximately to 25 percent of full-load. As a result, the water temperature in the evaporator 4 rises more rapidly than the temperature of the water in the superheater 5 because the flowrate in the superheater 5 is substantially greater. When the evaporation temperature corresponding to the working medium pressure is reached in the evaporator 4, steam will be generated therein. The generated steam, however, is condensed in the mixing vessel 20 by means of the feed water which is supplied via the duct 34. Thereafter, before evaporation takes place in the superheater 5, the valve 35 is rapidly closed and at the same time the pressure retaining element 32 is opened to such an extent that the pressure at the outlet of the superheater 5 is reduced to approximately atm. gauge. This pressure reduction is propagated through the superheater 5 into the evaporator 4 and causes an intensive evaporation of the feed water therein. The resultant steam surge expels the water from the superheater 5 and, owing to the large pressure difference and the large amount of steam suddenly produced, there is no danger of water stagnating in the individual tubes of the superheater 5. In further operation, the firing rate and the supply of feed water into the evaporator 4 is increased, the pressure at the outlet end of the superheater being raised by appropriate operation of the pressure retaining element 32.

Owing to the large excess of air, the thermal loading of the superheater 5 is reduced. A further advantage of the large excess of air is that the gas turbine assembly 27, 10 runs freely without the supply of energy even after a very short time. The direct supply of a large quantity of feed water to the superheater 5 during the first phase of start-up prevents damage to the superheater 5 by excessive temperatures of the hot flue gases which are carried a considerable way through the combustion chamber 3 by the high excess of air.

Referring to FIG. 2, wherein like reference characters indicate like parts as above, a steam generator which is constructed without a gas turbine assembly is provided with a liquid separator 40 instead of a mixing vessel 20, as above. A duct 41 is connected to the liquid separator 40 to draw off water and to direct the withdrawn water to a degasser 16. A line 34 branches off from the feed water line 11 and leads to a point between the exit of the steam from the separator 40 and the superheater 5 into the steam line. The starting up method for this steam generator is the same as described above with respect to the steam generator of FIG. 1.

Referring to H6. 3, wherein like reference characters indicate like parts as above, a steam generator which is also constructed without a gas turbine assembly has a line 42 which branches from the water chamber of the separator 40. This line 42 is constructed as a circulating line and joins the tube system of the steam generator upstream of the evaporator 4. A circulating pump 43 is also provided immediately upstream of the evaporator 4 so that the working medium which is separated in the separator can be constantly circulated through the evaporator 4. Alternatively, the circulating pump 43 may be disposed in the line 42 itself. An injection line 44 is also provided which connects into the tube system between two superheaters 5a, 5b and branches off from the line 34. The injection line 44 is provided with a valve 45 whose open cross section is adjusted in dependence upon the temperature of the steam discharged from the superheater surface 5b via a suitable control means 46, as is known. The starting up method for this system is also the same as described with reference to the steam generator of FIG. 1, the circulating pump 43 being switched on from the beginning. Circulation of the water from the separator 40 reliably avoids overheating of the evaporator 4. The valve 45 in the injection line 44 is closed during the starting up procedure.

What is claimed is:

1. A method of starting up a steam generator having an evaporator, a superheater including a plurality of tubes disposed in parallel with respect to the flow of working medium a bypass line connected to the outlet of the superheater an a pressure maintaining element in the bypass line; said method comprising the steps of initially supplying a first flow of feed water to the steam generator for flowing through the evaporator and superheater to the bypass line; I supplying an additional flow of feed water at a rate of flow at least 30 percent larger than the rate of said first flow to the superheater while bypassing the evaporator, said additional flow being supplied from a time no later than when a fire is ignited in the steam generator; and subsequently suddenly and substantially reducing said additional flow to the superheater while simultaneously opening the pressure maintaining element to greatly diminish the pressure at the outlet end of the superheater.

about 15 percent of full-load feed water flow rate.

3. A method as set forth in claim 2 wherein said additional flow rate is about 35 percent of full-load feed water flow rate.

4. A method as set forth in claim 1 which further comprises the steps of supplying the steam generator with fuel at about 15 percent full-load fuel flow rate and with combustion air at about 25 percent full-load combustion air flow rate during said initial step of supplying said first flow.

5. A method as set forth in claim 1 wherein the superheater is maintained at a full-load working pressure during said initial steps of supplying said flows of feed water.

6. A method as set forth in claim 5 wherein said full-load working pressure is atm. and wherein said pressure is reduced to 10 atm. upon opening of the pressure maintaining element.

7. In combination with a steam generator having an evaporator, a superheater including a plurality of tubes disposed in parallel with respect to the flow of working medium, a feed inlet for supplying feed water to said evaporator, a bypass line connected to an outlet of said superheater, and an adjustable pressure maintaining element in said bypass line; an additional feed water line connected between an outlet of said evaporator and an inlet of said superheater to supply an additional flow of feed water at least equal to 20 percent of the full-load feed water flow rate to the superheater.

8. The combination as set forth in claim 7 wherein said additional feed water line includes an adjustable valve therein for controlling the rate of flow of feed water therein and a mixing vessel therein downstream of said valve, said mixing vessel being connected to said outlet of said evaporator to receive a flow of working medium therefrom.

9. The combination as set forth in claim 7 which further includes a liquidseparator connected between said evaporator and superheater to convey working medium therebetween a line connecting said liquid separator to said superheater to direct a flow of working medium thereto and wherein said additional feed water line is connected to said line.

10. The combination as set forth in claim 9 which further includes a pair of said superheaters, a line connecting said superheaters in series, an injection line for feeding feed water from said additional feed line into said line between said superheaters, a control means in said injection line for adjusting the flow of feed water therethrough in response to the temperature of working medium passing from said superheaters, and a branch line between said liquid separator and an inlet of said evaporator for circulating working medium between said evaporator and said liquid separator.

2. A method as set fgglmggign l wherein said first flow is 

1. A method of starting up a steam generator having an evaporator, a superheater including a plurality of tubes disposed in parallel with respect to the flow of working medium, a bypass line connected to the outlet of the superheater, and a pressure maintaining element in the bypass line; said method comprising the steps of initially supplying a first flow of feed water to the steam generator for flowing through the evaporator and superheater to the bypass line; supplying an additional flow of feed water at a rate of flow at least 30 percent larger than the rate of said first flow to the superheater while bypassing the evaporator, said additional flow being supplied from a time no later than when a fire is ignited in the steam generator; and subsequently suddenly and substantially reducing said additional flow to the superheater while simultaneously opening the pressure maintaining element to greatly diminish the pressure at the outlet end of the superheater.
 2. A method as set forth in claim 1 wherein said first flow is about 15 percent of full-load feed water flow rate.
 3. A method as set forth in claim 2 wherein said additional flow rate is about 35 percent of full-load feed water flow rate.
 4. A method as set forth in claim 1 which further comprises the steps of supplying the steam generator with fuel at about a 15 percent full-load fuel flow rate and with combustion air at about 25 percent full-load combustion air flow rate during said initial step of supplying said first flow.
 5. A method as set forth in claim 1 wherein the superheater is maintained at a full-load working pressure during said initial steps of supplying said flows of feed water.
 6. A method as set forth in claim 5 wherein said full-load working pressure is 100 atm. and wherein said pressure is reduced to 10 atm. upon opening of the pressure maintaining element.
 7. In combination with a steam generator having an evaporator, a superheater including a plurality of tubes disposed in parallel with respect to the flow of working medium, a feed inlet for supplying feed water to said evaporator, a bypass line connected to an outlet of said superheater, and an adjustAble pressure maintaining element in said bypass line; an additional feed water line connected between an outlet of said evaporator and an inlet of said superheater to supply an additional flow of feed water at least equal to 20 percent of the full-load feed water flow rate to said superheater.
 8. The combination as set forth in claim 7 wherein said additional feed water line includes an adjustable valve therein for controlling the rate of flow of feed water therein and a mixing vessel therein downstream of said valve, said mixing vessel being connected to said outlet of said evaporator to receive a flow of working medium therefrom.
 9. The combination as set forth in claim 7 which further includes a liquid separator connected between said evaporator and superheater to convey working medium therebetween a line connecting said liquid separator to said superheater to direct a flow of working medium thereto and wherein said additional feed water line is connected to said line.
 10. The combination as set forth in claim 9 which further includes a pair of said superheaters, a line connecting said superheaters in series, an injection line for feeding feed water from said additional feed line into said line between said superheaters, a control means in said injection line for adjusting the flow of feed water therethrough in response to the temperature of working medium passing from said superheaters, and a branch line between said liquid separator and an inlet of said evaporator for circulating working medium between said evaporator and said liquid separator. 